Corrugated board can be manufactured in many different widths and thicknesses. The thickness of the board is determined by the number of medians and liners in the board. First, corrugations or flutes are created in a paper median by passing the median through a corrugator. Then, an alternating series of paper liners and paper medians, with an adhesive between each layer, are brought together in a moving web to form a board of desired thickness. The moving web passes through an assembly line that includes a hotplate section, where heat and pressure are applied to dry the adhesive, and a cooling section, where the board is cooled. The moving web is then cut and scored to make board of different shapes and sizes for boxes and other items.
Uneven moisture content in the source paper, which can cause portions of the board to shrink after the adhesive has set, is the principle cause of warping and the resulting waste encountered in manufacturing corrugated board. Accordingly, it is important to dry the source paper evenly before the medians and liners are brought together to form the board. To prevent warping, the source paper is passed through a preheater assembly that dries the source paper before it is processed by a machine for manufacturing corrugated board.
The preheater assembly typically includes a continuous web feeder, such as an unroller for feeding a continuous web of paper from a long sheet of paper wound around a core, a dryer, and a tension roller located between the feeder and the dryer for taking any slack out of the paper web. The dryer typically includes a large heated drum and two smaller idler rollers that keep the paper web in contact with the heated drum over a substantial portion of the circumference of the heated drum. From the dryer, the paper web travels to the machine for manufacturing corrugated board. Typically, the next stage in the assembly line is either a corrugator section, which flutes a paper web to create a median, or a single-facer section, which places a layer of adhesive between a median and a liner.
In order for the preheater to dry the paper properly, the paper web must be held tightly against the heated drum. If slack develops in the paper web, bubbles can form between the paper and the heated drum resulting in uneven moisture content in the paper exiting the dryer. Preventing slack from developing in the paper can be difficult, however, because the paper may have been rolled onto the core with uneven tension across the length of the paper roll. Typically, this causes one edge of the paper web to be taught while the other edge develops slack. This slack can remain in the web after it travels over the tension roller, over the first idler roller, and onto the heated drum, causing bubbles or loose edges to develop between the web and the drum.
In one conventional moving-web slack-reducing system designed to overcome this problem, the paper web is fed over a vertically-actuated tension roller. The vertical actuators, one typically placed at each end of the tension roller, allow each end of the tension roller to be lifted and lowered a small amount in an attempt to take any slack out of the paper web. But this solution is somewhat wanting in performance because removing the slack from the web requires precise positioning of the actuators. In addition, the tension tends to vary quickly across the paper web as the paper is unrolled. Removing the slack from the web under these conditions therefore requires fairly rapid and precise adjustments of the actuators, which are difficult to achieve at a reasonable level of investment.
Therefore, there is a need for a preheater assembly for a machine for manufacturing corrugated board that results in a paper web leaving the preheater assembly with a relatively even moisture content. There is a further need for a preheater assembly that removes the slack from a paper web without having to rely on fairly rapid and precise adjustments of a vertically-actuated tension roller.